US-20260126283-A1 - OPTICAL PATH LENGTH SENSOR

US20260126283A1US 20260126283 A1US20260126283 A1US 20260126283A1US-20260126283-A1

Abstract

An optical path length sensor for sensing a physical quantity of an external source includes a plurality of lasers, each having an optical resonator and a gain medium to produce a laser beam in the optical resonator. At least one of the optical resonators is configured to modulate the optical frequency of the laser beam when exposed to the external source. The sensor further includes a common carrier, in which the optical resonators are arranged, and a device configured to receive light from the plurality of lasers and to determine a difference between the optical frequencies of the laser beams. In another aspect the optical path length sensor includes a plurality of interferometers, each being an asymmetric Mach-Zehnder or an asymmetric Michelson interferometer, at least two of the plurality of interferometers having a different optical path length imbalance.

Inventors

Per GRÖN

Assignees

Lumiary AB

Dates

Publication Date: 20260507
Application Date: 20230927
Priority Date: 20221006

Claims (14)

1 . An optical path length sensor for sensing a physical quantity of an external source, comprising: a plurality of lasers, wherein each of the lasers comprises an optical resonator and a gain medium to produce by means of a pump source a laser beam in the optical resonator, wherein at least one of the optical resonators is configured to modulate the optical frequency of the laser beam in said at least one optical resonator when exposed to the external source; a common carrier, in which the optical resonators are arranged; and a device configured to receive light from the plurality of lasers and to determine a difference between the optical frequencies of the laser beams.
2 . The sensor of claim 1 , wherein the optical resonators include a first and a second optical resonator arranged on the common carrier, wherein the first and/or second optical resonator is/are configured to modulate the optical frequency of the respective laser beam when exposed to the external source and/or wherein the sensor comprises an open chamber receiving the first and the second optical resonator, one or more closed chambers receiving the first and the second optical resonator, or an open chamber receiving the first optical resonator and a closed chamber receiving the second optical resonator.
3 . The sensor of claim 1 , wherein each optical resonator comprises optical elements for redirecting light in the respective optical resonator, wherein preferably each of the optical element is a mirror or a waveguide.
4 . The sensor of claim 3 , wherein the at least one of the optical resonators is a first optical resonator comprising a first optical element and a second optical element, which have at least one of the following features A1) to A6): A1) the first optical element is movably or immovably arranged with respect to the second optical element, A2) the first optical element is arranged on a deformable element, in particular a cantilever or membrane, A3) the second optical element is arranged on a photonic integrated circuit, A4) the second optical element is configured as an output coupler for the laser beam, preferably the second optical element is placed on top of a vertical coupler of a photonic integrated circuit, A5) the first optical element and the second optical element are arranged in a closed chamber, which preferably includes a vent and/or inlet for a medium as external source, A6) the first optical element is a mirror including a plane or curved surface and/or the second optical element is a mirror including a plane or curved surface.
5 . The sensor of claim 3 , wherein one of the optical resonators other than the at least one of the optical resonators is a second optical resonator comprising a third optical element and a fourth optical element, which have at least one of the following features B1) to B6): B1) the third optical element is movably or immovably arranged with respect to the fourth optical element, B2) the third optical element is arranged on a deformable element, in particular a membrane, B3) the fourth optical element is arranged on a/the photonic integrated circuit, B4) the fourth optical element is configured as an output coupler for the laser beam, preferably the fourth optical element is placed on top of a vertical coupler of a/the photonic integrated circuit, B5) the third optical element and the fourth optical element are arranged in a/the closed chamber, which preferably includes a vent and/or inlet for a medium as external source, B6) the third optical element is a mirror including a plane or curved surface and/or the fourth optical element is a mirror including a plane or curved surface.
6 . The sensor of claim 1 , which includes at least one of the following features C1) to C6): C1) at least one pump source for optically pumping one or more of the lasers, C2) an optical pump source for emitting a beam of light that is split and used to pump at least two lasers, C3) at least one pump source for electrically pumping one or more of the lasers, C4) an electrical current source that is configured to pump at least two lasers, C5) at least one of said gain media is one of a semiconductor gain medium, in particular a gain medium for forming a semiconductor disk laser, or a solid-state gain medium, in particular a solid-state crystal, C6) the plurality of lasers are configured to be tunable.
7 . The sensor of claim 1 , wherein the carrier comprises a first layer, a second layer and side walls arranged therebetween for forming a hollow structure, wherein each optical resonator of the plurality of optical resonators comprise a first mirror arranged on the first layer and a second mirror arranged on the second layer, wherein preferably the first mirrors and the second mirrors each comprise a reflective coating on a substrate.
8 . The sensor of claim 1 , wherein the optical resonators are configured to act as a rigid body, and, in use, at least one beam path inside at least one optical resonator passes through a gas or liquid medium, or the optical resonators comprise a mirror embedded on a membrane or cantilever that, in use, moves when it is exposed to sound or changes in ambient pressure, and/or at least one of the optical resonators comprises a mirror embedded on a cantilever that is configured to move when it is exposed to an external acceleration or rotation force.
9 . The sensor of claim 1 , wherein the device for determining a difference between the optical frequencies comprises a plurality of interferometers for receiving light from the lasers, wherein each of the interferometer is an asymmetric Mach-Zehnder or asymmetric Michelson interferometer and is provided with an output optical coupler having a plurality of output ports, wherein at least two of the plurality of interferometers have a different optical path length imbalance; a plurality of photodetectors, each coupled to one of the output ports; and electronic processing circuitry for receiving signals from the plurality of photodetectors to compute the optical frequency of each of the laser beams.
10 . An optical path length sensor for sensing a physical quantity of an external source, comprising: at least one laser, which is configured to emit a laser beam having an optical frequency, wherein the at least one laser is configured to modulate the optical frequency of the laser beam when exposed to the external source; a plurality of interferometers for receiving light from the at least one laser, wherein each of the interferometer is an asymmetric Mach-Zehnder or an asymmetric Michelson interferometer and is provided with an output optical coupler having a plurality of output ports, wherein at least two of the plurality of interferometers have a different optical path length imbalance; a plurality of photodetectors, each coupled to one of the output ports; and electronic processing circuitry for receiving signals from the plurality of photodetectors to compute the optical frequency of the laser beam.
11 . The sensor of claim 10 , which has at least one of the following features D1) to D7): D1) the at least two of the plurality of interferometers have each two arms with a different optical length, the difference between the two arms defining the optical path length imbalance, wherein the ratio of the optical path length imbalances of the at least two interferometers is at least 10:1, preferably at least 100:1, most preferably at least 1000:1, D2) the at least two of the plurality of interferometers comprise an optical delay line to provide for the optical path length imbalance, preferably the optical delay line being provided by a fiber or a waveguide in a photonic integrated circuit, most preferably the optical delay line being provided by a coiled fiber or a spiral waveguide in a photonic integrated circuit, D3) the at least two of the plurality of interferometers serve each as a filter whose response with regard to the phase shift as a function of the frequency has a filter period, wherein the at least two of the plurality of interferometers have a different filter period, wherein the ratio of the filter periods is at least 10:1, preferably at least 100:1, most preferably at least 1000:1, D4) each output optical coupler has at least three output ports, preferably at least four output ports, D5) the at least two of the plurality of interferometers are arranged optically in parallel and connected to an input optical coupler device for receiving light from the at least one laser, D6) the at least one laser is one of a semiconductor laser or a solid-state laser, D7) at least one optical amplifier is provided for amplifying light to be received by one or more of the interferometers, preferably the optical amplifier being a fiber amplifier or a semiconductor optical amplifier.
12 . The sensor of claim 10 , wherein the interferometers include waveguides in a photonic integrated circuit.
13 . The sensor of claim 10 , comprising at least two lasers, wherein the electronic processing circuitry is configured to compute a difference between the optical frequency of the at least two lasers.
14 . The sensor according to claim 1 , which is configured to sense one or more of the following physical quantities: pressure, sound, ultrasound, displacement, temperature, force, acceleration, rotation force, voltage, an electric field, refractive index, concentration of one or more specific chemicals, in particular the sensor is a microphone for sensing sound in a given range, which preferably includes 20 Hz to 20 kHz.

Description

FIELD OF THE INVENTION This invention relates to techniques for sensing by means measuring optical path length, in particular to an optical path length sensor for sensing a physical quantity of an external source. Such an external source may effect e.g. a displacement to be measured, for instance in the microscopic scale or smaller, or a change in a refractive index. An example may include a pressure sensor or a microphone. BACKGROUND Interferometry is a widely used method for measuring optical path length. It is used in many applications including sensing of displacement, temperature, pressure, sound, ultrasound, concentration of gasses and chemical substances in various media, and aerosolized particles. Interferometric sensors provide improved resolution, but there are limits to how well they can perform. When optical power is limited, which is common for example in battery powered devices, the resolution of many interferometers is fundamentally limited by optical shot noise. The resolution limit of interferometry caused by optical shot noise is not an inherent limit of nature; it is possible to achieve higher resolution using engineered quantum states of light, such as with squeezed light or entanglement. Because of the fragile nature of these exotic states, it is difficult to achieve large improvements in resolution with this approach, and these techniques have so far seen only limited use in commercial applications. With shot noise limited measurements, the resolution increases with the square root of optical power. This is worse than the ultimate resolution limit for sensors as dictated by quantum physics, the Heisenberg limit, which is that the resolution of a sensor can increase at most at the same rate as the number of times the measured object is interrogated (for example, the number of times a photon bounces off a mirror). SUMMARY OF THE INVENTION It is an object of the invention to provide for an optical path length sensor, which has improved sensitivity. This object is achieved by the sensor as defined in claim 1 or 10. The further claims specify preferred embodiments of the sensor. According to a first aspect there is provided a sensor, which comprises a plurality of lasers, each of which comprises an optical resonator, and a common carrier, in which the optical resonators are arranged. Preferably, each of the plurality of lasers comprises a gain medium placed within the optical resonator to produce by means of a pump source a laser beam in the optical resonator. At least one of the optical resonators is configured to modulate the optical frequency of the laser beam in the optical resonator when exposed to the external source. The provision of a plurality of lasers has the advantage that laser beams with different optical frequencies can be produced, which can be compared to improve the measuring accuracy. The provision of a common carrier has the advantage that unwanted noise effects the optical resonators in the same way. The sensor may be configured such that its resolution is significantly increased without a quadratic increase in power consumption as is required in shot noise limited schemes, and enables a design which is compact and which may be fabricated at low cost. Preferably, the common carrier of the sensor is configured to mechanically couple the optical resonators such that unwanted noise appears as a common mode signal in the frequency of the plurality of lasers. Preferably, the sensor is configured to sense a physical quantity varying in time within a frequency range, wherein the carrier, which holds the optical resonators in place, is free of a mechanical resonance frequency that overlaps with the frequency range. An optical resonator may comprise a deformable element whose lowest resonance frequency is higher than the upper limit of the frequency range to be sensed. Preferably, the carrier forms a rigid body. An optical resonator may be configured such that part of it, e.g. a cantilever or membrane, is movable relative to the carrier. The carrier may be fabricated from a material that has a Young's modulus exceeding 60 GPa. Preferably, the common carrier of the sensor comprises a structure with a first layer and a second layer, wherein each optical resonator of the plurality of lasers comprises a first mirror arranged on the first layer and a second mirror arranged on the second layer. In one embodiment, the optical resonators are configured such that all of the laser beams are no more than 10 mm, preferably no more than 2 mm, from another laser beam. In one embodiment, the carrier element comprises a first parallel plate and a second parallel plate held together with side walls, forming a hollow structure, wherein each of the optical resonators comprise a first mirror and a second mirror. The first mirror and the second mirror each comprise a reflective coating on a substrate, wherein the substrate of all of the first mirrors forms the first parallel plate and the substrate